Gerald;
As you don't want a pulse train of that rate you shouldn't use counters and as that frequency is to high for you to make function calls in Software to change the line states, the only way out would be to generate a fixed digital pattern on a board which allows digital pattern generation.
You can refer to either the PCI-DIO-32HS or PCI-6534 for that purpose. With those boards, you can specify the digital pattern to be output on a specific line, and depending on the frequency you need you can specify the pattern in a way that the digital lines turn on and off with a fixed frequency.
Hope this helps.
Filipe